Our European Pat. 108590 discloses a method and apparatus for installation of transmission lines within tubular ducts using fluid drag to advance the transmission line along the duct. In the preferred form of apparatus described in that specification compressed gas is used to provide the fluid drag and an optical fibre transmission line package is introduced via a feed head and feed wheels are provided to urge the fibre package into the duct against the hydrostatic potential that is experienced. Once a sufficient length of the fibre package has been inserted the fluid drag on the inserted fibre overcomes the resistance of the hydrostatic potential and the fibre is propelled by fluid drag along the duct. During such an installation procedure there is substantially continuous relative motion between the duct and the fibre package.
For in-field applications it is desirable to have mobile equipment that can easily be moved from location to location, and can be utilised in confined spaces: this is particularly important for repair work which may have to take place in remote and awkward circumstances. With the apparatus described in E.P 108590 it has been found that the feed wheels and associated drive mechanism limits the minimum size of the equipment.
A further problem that arises with positive pressure feed heads is the provision of a seal to inhibit backflow of fluid between the inlet tube for the fibre package and the fibre package itself. It is necessary for the fibre to be able to advance without being over-tensioned, but at the same time the greater than atmospheric pressure within the feed head needs to be maintained. Sealing may be particularly difficult to achieve because there can be considerable variation in cross-sectional shape and diameter of some transmission line packages, for example the tolerance on the diameter of a multi-fibre package maybe as much as .+-.10%. Thus if a very closely fitting collet is located around the fibre package the positive deviations in diameter may result in the package being gripped too tightly, resulting in excessive tension on the optical fibre. In practice an inefficient sealing arrangement of a collet of greater diameter than the package and sufficient to accommodate the package cross sectional variation is used, and this results in fluid propellant leakages as high as 50% occurring between the fibre and collet.